ignore me coming back after 3 or so days of absence...
I believe it's either for a cool effect you can make by taking advantage of the fact that the video will always replay itself when it ends, or it's for viewer engagement, so they are going to watch it again just to see how cool the loop is.
Though sometimes the loop can be cool, it also doesn't really make sense in most cases. I just normally watch a short and after the shorts creator finishes what they have to say, they out of nowhere say something like "because..." and the video loops itself and the sentence is completed with what the person said at the start of the video. And normally that sentence, when taken out of context doesn't make sense on its own.